chore: Add helper scripts to download stunner tools

[levaitamas]

This PR adds helper scripts to download future turncat and stunnerctl binaries from github release assets. The scripts work on Linux and MacOS systems. Windows users we are sorry, however PRs welcome.

Currently we have no stable release with binaries available. The below sed oneliners edit the scripts to download test binaries from my fork:

sed -i "s|REPO=https://github.com/l7mp/stunner|REPO=https://github.com/levaitamas/stunner|" cmd/downloadStunnerCtl.sh

sed -i "s!REPO=https://github.com/l7mp/stunner!REPO=https://github.com/levaitamas/stunner!" cmd/downloadTurncat.sh

Acknowledment: scripts are baseod on downloadIstioctl. Thanks Istio devs!

[coveralls]

Pull Request Test Coverage Report for Build 7870660185

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.429%

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Totals
Change from base Build 7818148938:	0.0%
Covered Lines:	965
Relevant Lines:	1351

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💛 - Coveralls

[rg0now]

Thanks a lot, this is super-useful!